Sshd

sshd is the Secure Shell Daemon, the "server" program that allows ssh logins to your system. The config file is usually located in /etc/ssh/sshd_config (watch out for those underscores) and it's part of the base system.

In the configuration of sshd, you should restrict it to use only protocol version 2, as there have been insecurities found with sshv1.

Also using ssh keys instead of passwords can be an extra level of security.